90 degree hooks in bundled bars 2

[WARose]

In ACI 318-11 it says this:

R12.4.1....The development of bundled bars by a standard hook of the bundle is not
covered by the provisions of 12.5.

I cannot find where it is covered. Does this mean it is prohibited....or that it simply isn't covered?

If the answer is the latter....does anyone know if AASHTO covers it? Thanks.

 

[KootK]

I'm not aware of an explicit prohibition on using hooked, bundled bars but my feeling is that it's probably not allowed. And that, simply because it probably hasn't been tested. I think that you could use the equivalent bar area to suss out concrete splitting etc but, for the portion of the resistance that is inside corner concrete bearing, I don't think that would be valid.

 

[WARose]

Thanks Kootk. I was hoping you would reply because you use a variety of codes (some of them in other countries).

 

[KootK]

No sweat. I know that you like to keep your extraneous background information sparse but I'd love to know what's necessitating this condition. Development of the primary corbel-ish bars on that 2' deep pile cap thing?

 

[BridgeSmith]

The AASHTO code doesn't address it directly but indicates the development length modification factors for bundled bars should be calculated for a single bar of equivalent area as the bundle. Basic development length is taken as that for the individual bars, increased by 20% for 3 bar bundles and 33% for 4 bar bundles. If the hooks are together (turned in the same direction), I would calculate the hook length and embedment as if the bundle was a single bar of the equivalent area. If the hooks are splayed out, I would use the hook length for the individual bars, but I'd probably calculate the embedment using the total area, since there would be some overlap in the failure zones of the individual bars.

 

[WARose]

.... I'd love to know what's necessitating this condition. Development of the primary corbel-ish bars on that 2' deep pile cap thing?

No, not that thing. (Same project....different headache.) I was trying to develop some moment strength beyond a cut off point. (With not much space left.)

I might try some sort of arrangement (i.e. rotating the bars) where they aren't so close to each other.

 

[WARose]

thanks Hotrod.

 

[bones206]

Have you looked at headed bars? Shorter development length for larger bars — might get you out of bundling.

 

[cliff234]

In my opinion engineers should avoid bundling bars like the plague. Splices must be staggered and wrapping column ties around bundled bars can be a challenge.

But if you are hooking the bars, that implies you are at the top of the column. Why do you need bundled bars at the top of the column (where the loads are usually relatively small)?

It would be better to increase the compressive strength of the concrete (most strength comes from the concrete, not the steel), use a bigger column, or use ASTM Grade 80 reinforcing steel. I'm not sure what the specifics of your project are, but we always specify grade 80 bars for #11's and bigger on projects with more than 100 tons of #11 bars.

Do you need the hooks? Are you cranking moment into the column? And are all of the bundled bars required to get the moment into the column? What size are the bundled bars? If you really need that much steel, hopefully you are not bundling small bars. Can you reconfigure the bars so that you can eliminate the bundles by using bigger bars. If the bars are not bundled and you need to develop them at the connection to the slab, you could consider using Lenton Terminators.

(I realize that I am not answering your question - but rather I am offering solutions to your problem - because hooking bundled bars is not a good idea.)

 

[WARose]

But if you are hooking the bars, that implies you are at the top of the column. Why do you need bundled bars at the top of the column (where the loads are usually relatively small)?

It's not a column, it's a (thick) heavily reinforced slab.

It would be better to increase the compressive strength of the concrete (most strength comes from the concrete, not the steel),....

For flexural reinforcement? I don't think so. Upping the f'c has a almost negligible effect.